Factors influencing mining method selection

Note: Oil and gas deposits are not discussed in this article. Underground mining methods are the focus of this article.

The selection of a mining method for an ore deposit is based on many factors that are driven by the economics and profitability of the mine for a company. These include the ore grade and recovery, cost of infrastructure, ore extraction, labour and machine costs, and underground support costs. The characteristics of the orebody itself form the basis for these decisions, including the thickness and orientation of the mineralization, the ore and rock strength, the distribution of mineralization within the orebody, the geotechnical environment, and the depth of mineralization and surface conditions. In some cases, these conditions change in a single mining operation. If significant enough, a change in mining method in one ore deposit can occur.

Geotechnical considerations when selecting a mining method are a relatively recent trend, caused by the increased dimensions and production rates required of mining operations in order to meet growing expectations of profitability. Since these larger projects require a longer period of satisfactory performance in terms of ore recovery and ground support, more formal and rigorous methodologies are necessary in mine design (Brady and Brown, 2006). Geotechnical factors include in-situ mechanical properties of the orebody and country rocks, the geological structure of the rockmass, the ambient state of stress and the hydrogeological considerations in the zone of potential mining influence (Brady and Brown, 2006). The goals of geotechnical consideration in mine design, regardless of the mining method, are to:

• Ensure the overall stability of the complete mine structure, defined by the main orebody, mined voids, ore remnants (pillars) and adjacent country rock;
• To protect the major service openings and infrastructure throughout their design life;
• To provide safe access and working places in and around the centres of ore production; and
• To preserve the mineable condition of unmined ore reserves (Brady and Brown, 2006).

Mining methods have evolved significantly in the last several decades as improvements have been made on machinery used to extract the ore, understanding and experience with the behaviour of the rockmass and underground stresses has developed, and as newly discovered ore bodies are located in increasingly difficult conditions.

Mining Method Classification

From a geomechanical perspective, mining methods can be classified based on the type and degree of support required in mining operations. Supported mining methods include open stoping and room-and-pillar mining, where natural support is provided by ore remnants (e.g. pillars), or cut-and-fill stoping and shrinkage stoping, where support for the walls of the void remaining after ore extraction is provided by backfill or by fractured ore temporarily retained in contact with mined stope walls. Cave mining methods include block caving and sublevel caving, where no support is used because fragmented rock fills and flows through the stopes. A classification of underground mining methods, subdivided based on pillar supported and unsupported groups, is shown in Figure 1.

Figure 1: A hierarchy of underground mining methods and associated rockmass response to mining (Brady and Brown, 2006)

The distinction between these two broad categories of mining methods can be made by comparing the displacements induced in the country rock and energy redistributions in the rockmass caused by mining activities. Supported mining aims to restrict displacements in the country rock to elastic behaviour and prevent failure of the rockmass. The success of these methods depends on the ability of the near-field rockmass to sustain compressive stresses in order to maintain elastic behaviour. The mining issue therefore becomes prevention of unstable energy releases (e.g. rockbursts) associated with increased near-field stress, which could cause failure of support elements, sudden closure of stopes, or rapid fracture generation in the surrounding rock. A schematic of a supported mining method (room-and-pillar) is shown in Figure 2.

On the other hand, cave mining purposefully induces large displacements following fragmentation of the rockmass, resulting in energy dissipation in the caving rockmass. The success of this method depends on exploiting the discontinuous behaviour of a rockmass when confining stresses are relaxed (Brady and Brown, 2006). The mining issue here is to maintain steady displacement of the fragmented orebody so to prevent the development of unstable voids. The rate of slip and fragmentation of the rockmass must be proportional to the rate of ore extraction. A schematic of a caving mining method (block caving) is shown in Figure 3.

Figure 2: Schematic of a supported (room-and-pillar) method of mining (after Hamrin, 2001)

Figure 3: Schematic of a mechanized block caving operation method of mining at the El Teniente Mine, Chile (after Hamrin, 2001)

Geotechnical Factors of Underground Mining Methods

Artificially Supported

Bench-and-Fill Stoping

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Figure 4: Bench-and-fill stoping geometry in the Lead Mine, Mount Isa Mines, Queensland, Australia; (a) longitudinal section, and (b) cross-section (after Villaescusa, 1996)

Shrink Stoping

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Figure 5: Layout for shrink stoping (after Hamrin, 2001)

Unsupported

Longwall Mining

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Figure 6: Schematic of longwall mining in hard rock (after Hamrin, 2001)

Sublevel Caving

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Figure 7: Schematic of transverse sublevel caving (after Hamrin, 2001)

Summary

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Method Class	Method	Relative magnitude of displacements in country rock	Strain energy storage in near field rock	Suitable orebody geometry	Suitable orebody grade	Suitable orebody, country rock strength	Suitable depth
Pillar supported	Room-and-pillar	..	..	..	..	..	..
Pillar supported	Sublevel open stoping	..	..	..	..	..	..
Artificially supported	Cut-and-fill	..	..	..	..	..	..
Artificially supported	Bench-and-fill	..	..	..	..	..	..
Artificially supported	Shrink stoping	..	..	..	..	..	..
Artificially supported	VCR stoping	..	..	..	..	..	..
Unsupported	Longwall mining	..	..	..	..	..	..
Unsupported	Sublevel caving	..	..	..	..	..	..
Unsupported	Block caving	..	..	..	..	..	..

References

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• Bieniawski, Z.T. 1989. Engineering rock mass classifications. New York: Wiley.
• Brady, B.H.G. and Brown, E.T. 2006. Rock Mechanics for underground mining, 3rd Ed. The Netherlands: Springer.
• Brown, E. T. 2003. Block Caving Geomechanics. Julius Kruttschnitt Mineral Research Centre: Brisbane.
• Bullock, R. and Hustrulid, W. 2001. Chapter 3: Planning the Underground Mine on the Basis of Mining Method. In: Underground Mining Methods: Engineering Fundamentals and International Case Studies (eds W. A. Hustrulid and R. L. Bullock), 29-48. Society for Mining, Metallurgy and Exploration: Littleton, Colorado.
• Hamrin, H. 2001. Chapter 1: Underground mining methods and applications. In: Underground Mining Methods: Engineering Fundamentals and International Case Studies (eds W. A. Hustrulid and R. L. Bullock), 3–14. Society for Mining, Metallurgy and Exploration: Littleton, Colorado.
• Herne, V. and McGuire, T. 2001. Chapter 13: Mississippi Potash, Inc.’s, underground operations. In: Underground Mining Methods: Engineering Fundamentals and International Case Studies (eds W. A. Hustrulid and R. L. Bullock), 137-141. Society for Mining, Metallurgy and Exploration: Littleton, Colorado.
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• Krauland, N., Marklund, P.-I., and Board, M. 2001. Chapter 37: Rock support in cut-and-fill mining at the Kristineberg Mine. In: Underground Mining Methods: Engineering Fundamentals and International Case Studies (eds W. A. Hustrulid and R. L. Bullock), 325-332. Society for Mining, Metallurgy and Exploration: Littleton, Colorado.
• Laubscher, D.H. 1990. A Geomechanics Classification System for the Rating of Rock Mass in Mine design. Journal of the South African Institute of Mining & Metallurgy, vol. 90, no 10, pp. 257-273.
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• Marchand, R., Godin, P., and Doucet, C. 2001. Chapter 19: Shrinkage stoping at the Mouska Mine. In: Underground Mining Methods: Engineering Fundamentals and International Case Studies (eds W. A. Hustrulid and R. L. Bullock), 189-194. Society for Mining, Metallurgy and Exploration: Littleton, Colorado.
• Villaescusa, E. 1996. Excavation design for bench stoping at Mount Isa mine, Queensland, Australia. Trans. Instn Min. Metall., 105: A1–10.